(1) Field of the Invention
The present invention relates to a laminate cased battery including an outer casing made from a sheet of metal laminate, and more specifically, to the shape of the outer casing.
(2) Description of the Related Art
In recent years, portable machines such as mobile phones have become prevalent. In such portable machines, batteries are widely used as the power sources. Among those batteries, lithium polymer batteries with laminate outer casings (hereinafter referred to as “laminate cased batteries”), which have an advantage of being lightweight and having high energy density, are adopted in many types of portable machines.
The laminate cased batteries are formed by housing an electrode assembly, which includes a positive electrode plate, a negative electrode plate and a separator, into a dent that has been formed by executing a punch working on a sheet of metal laminate, bending the sheet so that a half of the sheet without the dent overlaps with the other half with the dent to close the opening of the dent, and sealing the folded sheet at the periphery of the dent. It should be noted here that many laminate cased batteries adopt what is called three-side-sealed structure in which the dent is rectangular, and the outer casing is formed by forming the dent in a sheet of laminate, bending the metal laminate sheet using one side of the dent as the return line, and sealing the folded sheet at three side edges surrounding the opening of the dent. Laminate cased batteries with such a structure are disclosed in, for example, Japanese Patent Application Publications No. 2000-208110 and No. 2003-303577.
However, in the laminate cased batteries with such a structure, when the dent is formed in the laminate sheet, pin holes or cracks often occur at corners of the dent in the laminate outer casing. It is thought that the pin holes or cracks will become a major problem especially when, in response to the increasing demands, the provision of a laminate cased battery with increased capacity or reduced weight is tried. Especially, when the capacity is increased, the size, including the thickness, of the electrode assembly will be increased, as well. In such a case, a deeper dent needs to be formed in the laminate cased battery. And the pin holes or cracks are more apt to occur when the deeper dent is formed.